The present invention is concerned with type train assemblies for high-speed printing machines now in wide use for printing information stored in computers; and is particularly directed to that type of such machines in which the type faces are mounted on carriers attached to an endless belt trained around a pair of drive wheels and passing rectilinearly through a printing station, at which hammers selectively produced impacts of printing paper and inked ribbon against the rapidly moving type faces. Machines of this nature should produce lines of print with reduced vertical misalignment, as compared with the type of machine which uses a drum having type faces spaced circumferentially around its periphery, because they use a horizontally moving type font as opposed to a vertically moving type font on a drum. Furthermore, it is much easier to change the type faces, since they are formed on slugs mounted on small individual carriers removably attached to the belt. A larger selection of characters can be made available merely by exchanging individual type slugs.
However, printing machines of the type train-and -belt kind have suffered from a relatively poor printing quality, have shown less reliability and durability, and have required more maintenance. As the print hammers strike the paper and ribbon against the type faces, they impose a retarding force that tends to drag the type and to stretch the belt, which must recover elastically between successive impacts occurring at extremely short intervals The necessarily-flexible character of the belt allows considerable freedon of movement of the type faces, both in the plane of these faces and in the direction of hammer impact. These factors combine to produce print that is usually neither very well aligned nor clear-cut. Another difficulty is that since the belt must travel at a high velocity, the mass of the carriers applies considerable centrifugal stretching force to the belt passing around the drive wheel, which subjects the belt and the carrier attachments to additional stress, and often even results in the type carriers becoming detached and flying off the belt.
It is the general object of this invention to provide an improved type train assembly and type carrier for printing machines, which produces better print quality. It is a further object to increase the durability and reduce the maintenance requirements of a type train assembly. It is another object to provide an improved type carrier which features improved means for secure attachment to a belt, and which is strong but light in weight, so that it may be made longer to carry more type faces and thereby reduce the number of carriers which must be attached to the belt to provide any required variety of type. Further objects and advantages of the invention will appear as the following description proceeds.
Briefly stated, according to a preferred embodiment thereof, I carry out my invention in part by employing a pair of parallel laterally spaced belts, to each of which is attached a series of type carriers located centrally between the belts. This balances the tractive load between the two belts, thereby not only dividing the strain produced by the drag of hammer impacts, but also resisting the tendency of the type to move irregularly and to twist in the plane of the type faces. The use of two belts also divides the belt tension, which increases their longevity.
I attach the type carriers to the belts in a mutually abutting train, which prevents the belts from stretching in the rectilinear span between the drive wheels through a printing station, and confines stretching to the relatively short curved spans around the wheels. The abutting ends of the type carriers are chamfered to avoid interference as they pass around the drive wheels. The belts are made somewhat short for the wheel spacing, and are stretched around the wheels so that they are held in tension. This tension is far greater than the drag caused by print hammer impacts, and combines with the abutting relation of the type carriers to improve the uniformity of motion of the type faces, thereby contributing to the quality of the printing.
A fixed guide bar is arranged to slide in grooves formed in rear faces of the type carriers, opposite the type faces. This firmly supports the type in the direction of hammer impact, and thereby improves the clarity of the print; furthermore, it holds the type faces in a fixed path through the printing station and prevents lateral displacement and twisting.
The individual type carriers include a central flange portion comprising a type block formed on one surface with the type faces, and a pair of laterally-entending flange portions including arms arranged on either side of the type block. The guide groove in the rear face of the carrier is defined by the rear face of the type block and between the laterally-extending flange portions. The carriers may be formed entirely of a light-weight material such as plastic, or the type block may be formed of metal and the flange portions of plastic. In the latter case, openings through the type block allow the laterally-extending flanges to be integrally molded to form a unit with the metal type block. The strength and lightness of this construction allows the carrier to be made longer than heretofore without excessive weight, so that a larger number of type faces can be formed on each carrier and the total number of carriers thereby reduced, decreasing machine cost and expediting the operation of changing or replacing type.
The laterally-extending flange portions of the carrier extend into arms which engage a toothed inner major surface of each belt, and are formed with recesses which conformably receive the belt teeth. The arms terminate in transverse flanges engaging the outer lateral edges of the belts, and in transverse projections which grip the outer major surface of each belt adjacent its outer edge. The laterally-extending flange portions also extend through a gap between the belts at which the type faces are presented, and are formed with further transverse projections which grip the outer major of each belt adjacent it inner lateral edge. The type carriers are thus held securely on the belts.